In a microwave radio communications system for example using QAM (quadrature amplitude modulation), it is well known to mix an IF (intermediate frequency) signal with an LO (local oscillator) signal at a microwave carrier frequency to produce an RF signal which is amplified in a power amplifier and transmitted. As is also well known, such a power amplifier generally has a non-linear transfer characteristic. In order to accommodate such a transfer characteristic it is known, for example from Putz U.S. Pat. No. 3,755,754 issued Aug. 28, 1973 and entitled "Predistortion Compensation for a Microwave Amplifier", to predistort the input to the power amplifier to compensate for the non-linearity.
The power amplifier non-linearity applies not only to the gain characteristic of the amplifier, but also to the signal phase, resulting in what is referred to as AM/PM (amplitude modulation to phase modulation conversion) distortion. This occurs in that signals passing through the power amplifier are subject to a phase change which is nonlinearly dependent upon the signal amplitude, so that an amplitude modulation of the input signal to the amplifier results in an amplified signal in which there is a phase modulation as well as an amplitude modulation.
It is known to adjust the phase characteristics of the predistorter manually in order to compensate as accurately as possible for the phase distortion of the power amplifier. However, such manual adjustment is undesirable, and can not conveniently accommodate changes, for example over time and with varying temperature, in the power amplifier characteristics. Accordingly, it is desirable to provide an adaptive control of the phase characteristics of the predistorter.
Such an adaptive control can be provided in a data directed manner by reverse signalling feedback from a receiver of the microwave radio communications system to control the transmitter phase. However, such reverse signalling feedback procedures fail to operate when the communications eye-pattern is closed, as typically occurs during signal fading. In order to avoid this problem it would be conceivable, but unduly expensive and hence commercially impractical, to provide a complete demodulator at the transmitter for adaptive control purposes.
An object of this invention, therefore, is to provide an improved method of adaptively controlling a predistorter to compensate for phase distortion in a power amplifier of an RF (radio frequency) communications system.